This invention relates to direct current motors or AC commutator (Universal) motors. More particularly, this invention relates to such motors which use a concentrated winding on the rotor with coils wound around the teeth.
In conventional DC motors or AC commutator (Universal) motors, there are three types of rotor armature windings: lap windings, wave windings and frog-legs windings. These windings are made with simple coil elements which are always interlocked. With an interlocked winding, the ratio between the axial length of the end-windings and the axial length of the armature magnetic circuit is relatively high as it is described by Klein U.S. Pat. No. 4,329,610, Ban et al. U.S. Pat. No. 4,197,475 and Ikeda U.S. Pat. No. 4,437,028.
All these windings differ primarily by the method which is used to connect the terminals of the simple coils to the commutator. A lap winding is also known as a multiple winding and for this kind of winding the number of parallel paths are equal to the number of poles. The wave winding is sometimes called a series winding and it has only two paths in parallel, regardless of the number of poles. The frog-leg winding is the association of a lap winding and a wave winding placed on the same armature, in the same slots, and connected to the same commutator bars.
The most significant problem with using a lap winding is that the voltages induced in the different parallel paths are unequal. These differences of induced voltages are due to unequal magnetic circuit reluctances or unequal fluxes under the different poles, which are created by rotor eccentricity, misalignment of the poles, and/or differences in permanent magnet magnetization. Because of the imbalance in induced voltages, circulating currents appear in the windings and through the brushes. These circulating currents cause unnecessary heating of the coils and brushes and tend to produce poor commutation.
The use of equalizer connections is the common solution to overcome the undesirable effects of circulating currents. These connections improve the current commutation and relieve the brushes of existing circulating currents by providing low resistance paths which by-pass the brush contacts. In a wave winding, the problem of the circulating currents due to the unbalanced voltages of the parallel paths is minimized but it is also impossible to get perfectly balanced voltages.
To avoid the interlocking of the coils, it is possible to directly wind the armature simple coils around each tooth of the rotor magnetic circuit. This kind of winding is called a concentrated winding, as described in our scientific papers, “Permanent Magnet Brushless DC Motor with Soft Metal Powder for Automotive Applications,” IEEE Industry Applications Society, St. Louis, October 1998, and “Synthesis of High Performance PM Motors with Concentrated Windings,” IEEE IEMDC, Seattle, May 1999. This kind of winding is also called a non-superposed winding, as described by Ban et al. U.S. Pat. No. 4,197,475. This kind of winding reduces the copper volume of the end-winding, the copper losses and the total axial length of the motor. The efficiency is improved when compared to the efficiency of classical structures. This winding structure is also easier to realize than a lap winding or a wave winding. When the axial length of the motor is small and the outside diameter of the motor is important, the use of such a winding structure allows a gain of 70% as compared to the volume of copper used in an overlapped winding.
Rotor structures with a concentrated winding have a small number of slots and the magnetic circuit is easier to realize. The magnetic circuit can be realized with a conventional soft magnetic laminated material (a yoke made of a stack of laminations) but it is also possible to use a soft magnetic composite material made of metal powder. The permeability of the soft magnetic composite is usually three times lower than the permeability of the conventional laminated materials like it is described by Jack et al. W.O. Pat No. 99,50949. This low value of permeability reduces the value of the coil inductances in the armature and the commutation process in both collector and armature is improved. A rotor structure with a small number of slots is also very well adapted to the realization of the armature magnetic circuit of direct current motors or ac commutator (Universal) motors with a soft magnetic composite material made of metal powder. With a small number of slots having relatively large dimension, the mechanical constraints on the direct molding process of the rotor yoke are reduced. It is also possible to easily insert the end-windings in the active part of the rotor magnetic circuit. This axial insertion of the end-windings improves the reduction of the volume of copper and the total axial length of the motor.
However, the concentrated winding technique is too often associated and restricted to windings with a short pitch, i.e. windings with lower performances than the performances of the classical winding structures. The concentrated windings with a short pitch are then limited to sub-fractional power applications (lower than 100 W) such as used in electrical motors for computer peripherals or toys. This is the case for the simplest and low cost brush direct current motor, which is widely used for toys. This 2-pole motor uses permanent magnets on the stator core, and has three teeth on its rotor core and a concentrated winding with one coil only wound around each tooth. The armature coil terminals are connected to a commutator with three segments and two brushes, as described by Fujisaki et al. U.S. Pat. No. 4,868,433. This structure has a winding with a short pitch of 120 electrical degrees. The winding coefficient or the ratio between the fundamental component of magnetic flux embraced by the winding and the total magnetic flux per pole is only equal to 0.866.
The main drawbacks of this motor structure are its low performance in terms of torque to weight ratio, torque ripple, and poor commutation performance if the power is increased. With this structure, the induced voltages in the coil paths between brushes are not always balanced. This unbalanced condition of operation produce supplementary losses, torque ripples, mechanical vibrations and commutation problems. These problems are acceptable for low power applications only.